The invention relates to a process for preparing sorbic acid. Various processes are known for preparing sorbic acid. A particularly economical process starts from the polymeric reaction product polyester which is prepared by reacting crotonaldehyde with ketene in the presence of a fatty acid salt of a divalent and/or trivalent metal of subgroup II to VIII of the Periodic Table of the Elements as catalyst (DE-A 1 042 573). The catalysts are generally used at 0.1 to 5%, preferably at 0.5 to 2%, of the rate of crotonaldehyde used. The polyester thus prepared can be converted into sorbic acid in various ways.
An industrially important process consists, for example, of the thermal catalytic cleavage of the polyester, the polyester being cleaved in an inert diluent which boils at above 150xc2x0 C. at atmospheric pressure (DE-A 1 059 899) in the presence of a secondary or tertiary aliphatic amine as catalyst at temperatures of 160 to 220xc2x0 C., simultaneously distilling off the sorbic acid and the solvent (DE-A, 1 282 645). The polyester is cleaved in the presence of 0.5 to 10, preferably 2 to 5, percent by weight of the catalyst amine. In the patent publication (DE-B 1 282 645), it is further maintained that a still greater amount of amine does not lead to a higher yield.
The invention relates to a process for preparing sorbic acid by thermal cleavage in a solvent of the polyester prepared from crotonaldehyde and ketene, which comprises cleaving the polyester in the presence of 20 to 100% by weight, based on the polyester used, of a secondary or tertiary aliphatic, alicyclic nitrogen- and/or oxygen-containing or aliphatic-aromatic substituted amine boiling at atmospheric pressure above 100xc2x0 C., preferably above 150xc2x0 C., as catalyst.
Surprisingly, it has been found that in the thermal cleavage of the polyester as described in DE-A 1 282 645, other than as described there, an abrupt increase in yield of sorbic acid is found when the concentration of the catalyst amine is selected to be at least twice as high as the concentration upper limit specified in DE-A 1 282 645. As described in DE-A 1 282 645, a slight further increase in the amount of amine above the upper amount of catalyst specified there does not lead to significant effect on the sorbic acid yield. Only a great increase in amine concentration leads to significant increase in yield.
Advantageously, the polyester is cleaved in an inert solvent which boils, at atmospheric pressure, above 150xc2x0 C., preferably above 180xc2x0 C., the mixture being heated to temperatures of 150 to 300xc2x0 C., preferably to 270xc2x0 C. (DE-A 1 059 899). The solvent is generally used in 1 to 15 times the amount by weight, based on the polyester. The thermal cleavage of the polyester advantageously takes place in the presence of 20%, preferably 20 to 60%, of a secondary or tertiary aliphatic, alicyclic, 5- or 6-membered heterocyclic nitrogen- and/or oxygen-containing or aliphatically aromatically substituted amine boiling at atmospheric pressure above 100xc2x0 C., preferably above 150xc2x0 C., as catalyst at temperatures of 160 to 220xc2x0 C., simultaneously distilling off the sorbic acid and the solvent.
Suitable amines which may be mentioned by way of example are: methyloctadecylamine, dimethyloctadecylamine, dimethylhexadecylamine, dimethyltetradecylamine, dimethyldodecylamine, dibutyidodecylamine, N,Nxe2x80x2,N,Nxe2x80x2-tetramethylhexamethylenediamine, N,N,Nxe2x80x2-trimethyl-Nxe2x80x2-phenylethylenediamine, N-octadecylpyrrolidone, N-octadecylpiperidine, N-dodecylmorpholine, N,Nxe2x80x2-dipropylpiperazine, xcex1-hexylpyrrolidone, triethylenetetramine, ethylbis(xcex2-ethylaminoethyl)amine, 1-octyldiethylenetriamine, ethylene glycol bis(2-methylaminoethyl ether), dioctadecylamine, diethylenetriamine, trioctadecylamine, trioctylamine, tricyclohexylamine.
To carry out the reaction, suitable diluents are aliphatic, alicyclic or aromatic hydrocarbons, their chlorine, bromine and nitro derivatives, and also ethers and silicone oils whose boiling point at atmospheric pressure is above 150xc2x0 C., preferably above 180xc2x0 C. However, ketones, esters, carboxylic acids and alcohols of the appropriate boiling range can also be used as diluents, although in general the results are not quite as good, since they apparently partly react with the reaction mixture. It is expedient to use those diluents or solvents which are liquid at ambient temperatures, boil at atmospheric pressure below 300xc2x0 C., preferably below 270xc2x0 C. and form azeotropic mixtures with sorbic acid, so that they can also act as an entrainer, such as petroleum fractions, dodecane, tetradecane, 5-methyidodecane, dodecene, dicyclohexylmethane, p-di-tert-butylbenzene, 1-methylnaphthalene, 2-methylnaphthalene, 1-ethyinaphthalene, tetrahydronaphthalene, biphenyl, naphthalene, halogenated aliphatic, cycloaliphatic or aromatic hydrocarbons, such as dichlorododecane, 1,5-dibromopentane, benzotrichloride, o- and m-dibromobenzene, nitro compounds such as nitrobenzene, 2-nitrotoluene, nitriles such as benzyl cyanide, carbonyl compounds such as acetophenone or the heterocyclic 2-acetylthiophene, heterocyclic compounds such as chromane, thiophene, ethers such as resorcinol dimethyl ether, diphenyl ether, safrole, isosafrole, acids such as enanthic acid, xcex1-ethylcaproic acid, caprylic acid, capric acid, esters such as ethyl benzoate, methyl phenylacetate and methyl salicylate (DE-A-1 059 899).
The examples below illustrate the invention. The starting material used is a polyester-containing reaction product which is obtained in a similar manner to German published application 1 042 573, example 1. In this case, 420 g of ketene are introduced into a stirred mixture of 800 g of crotonaldehyde, 1200 ml of toluene and 14.2 g of zinc isovalerate at a temperature between 25 and 35xc2x0 C. The excess crotonaldehyde and toluene are removed in vacuo. The residue obtained is 1150 g of polyester in the form of a high-viscosity brown liquid. In addition to the zinc content of 3000 ppm, this reaction product still has contents which cannot be converted into hexadienoic acids, such as diketene polymers and crotonaldehyde resins.
The fraction convertible into hexadienoic acids was determined by basic saponification of a solution of 60 g of sorbic polyester in 120 g of toluene by 33 g of potassium hydroxide in 260 g of water at room temperature. This produces in the aqueous phase potassium sorbate and the potassium salt of 3-hydroxy-4-hexenoic acid, from which hexadienoic acid can be obtained by acidification. By means of quantitative determination of the two reaction products using HPLC, the polyester fraction convertible into hexadienoic acids can be determined. By means of these mild conditions, the polyester content may be determined very much more precisely than as described in DE-B 1 282 645. Thus the fraction of the crude polyester which is convertible to hexadienoic acids is 89 to 90%, and not, as assumed in DE-B 1 282 645, only 80%. The yields achieved in DE-B 1 282 645 must therefore be corrected, see comparative example 1.